An interactionist-discovery theory of child phonology is proposed based on the following tenets: children invent their own phonological rules, and phonetic mastery is not automatically or generally in step with learning about phonemic contrasts. When a child learns the sound pattern of a language, there is constant interaction between the structure and that which is structured, and there is active discovery of the structure. Evidence is principally drawn from a recent longitudinal intensive case study, but independent support is also cited. Based on the case study, two modes of rule formation are discussed. The first rule invention is essentially a generalization process that can be interpreted as a carrying over of articulatory habits from one or a few established words to new words. In the second rule invention that occurred with the child, two different but similar output (surface) patterns coalesced or consolidated with one another. There were several different cases of rule formation by consolidation, and two of them are discussed. Evidence from the child concerning the early development of phonemic patterning is cited, and the phenomenon of the selective avoidance of certain sounds by some children is discussed. (SW)